Conventional measurement devices for vehicle speed need an additional independent reference speed in order to calibrate the measurement device. In US2006/0265112 A1, for example, a method and system for calibrating speed of a vehicle having plurality of wheels is disclosed. The method comprises the steps of sensing a first vehicle speed based on an average tire size associated with the plurality of wheels; sensing a second vehicle speed based on global positioning data; and automatically calibrating the speed of the first and second vehicle speed. The method of calibration is inaccurate as the first vehicle speed as well as the second vehicle speed are determined indirectly and are not independent.
Automotive speed over ground sensing based on self-mixing laser Doppler interferometry is expected to be of increasing importance, inter alia to improve ESP and other car safety systems.
However, to measure the speed-over-ground of a car, the orientation of the sensor with respect to the road surface influences the measurement. As well, velocity components laterally to the driving direction and vibrations influence the calculation of the velocity from the measured interferometric signals.
For car safety systems, the dynamics of the car is continuously being monitored. Currently, safety systems make use of the input of wheel sensors measuring the revolution of each single wheel, and several input signals from a central sensor box with e.g. a multi-axes accelerometer and a gyroscope. The kinetic data recorded is unfortunately incomplete. Currently, it is only possible to measure acceleration and rotation of the car bodywork together with the forward velocity of the car derived from the revolutions of the wheels. There are, however, no commercial sensor systems available which measure the velocity picked up by the car with respect to the road.
Such a measurement of the speed-over-ground of a car would (1) result in a direct instead of derived measurement of the forward velocity, and (2) give access to the lateral velocity of car. Especially the lateral velocity picked up by the car is a very useful control parameter for car safety systems making possible a considerable improvement of current safety systems.
In principle, several technologies could be employed to measure the speed-over-ground. For example, one could consider RADAR, camera-based image recognition techniques, or laser Doppler interferometry. However, these technologies need to be able to deal with the strongly varying conditions typically encountered in automotive (e.g. in heavy weather such as rain, or snow), have a manageable size, and be cost-effective. For the first time, a technology has been identified, namely laser sensors using the self-mixing interference principle, which boils down to laser Doppler interferometry, which fulfills all these automotive-specific requirements.
A SMI sensor incorporates a laser which is aimed at the road under a certain angle. When the laser beam hits the road, it will be reflected in all directions. When the road moves with respect to the laser (i.e. the car with the laser mounted thereon moving with respect to the road), the frequency of the reflected light is slightly different from the frequency of the incident laser beam. This frequency shift is the so-called Doppler shift and proportional to the component of the velocity of the road into the direction of the laser beam. When a small portion of this reflected, Doppler-shifted laser light re-enters the laser cavity, it will mix with the ‘undisturbed’ laser cavity light leading to an interference pattern. This interference pattern will change periodically with again exactly the Doppler frequency. These changing interference patterns inside the laser cavity lead to laser power fluctuations; in this way the Doppler frequency and thus the road velocity can be determined from the laser power.
In order to measure the two-dimensional velocity vector of the car with respect to the road, two laser beams are required. Also, the exact angles between the laser beams and the road, and the laser beams and the car need to be known in order to relate the Doppler frequency shift to the car's velocity.
The problem of applying Doppler interference to measure the speed-over-ground of a car lies within the fact that for the velocity measurement the exact orientation of the sensor with respect to the road and the sensor with respect to the heading direction must be known. However, since the laser sensor is mounted to the car bodywork, its orientation, and thus the angles of the incident laser beams, will change continuously due to suspension vibrations. Simulations have shown that a small deviation of the equilibrium suspension situation of 5 centimeters leads to an unacceptable measurement error. However, action of the suspension system is unpredictable and thus it leads to unacceptably low measurement accuracy. The same also holds for a misalignment of the sensor with respect to the forward or heading direction.